1. Field of the Invention
The present invention relates to metal sheet materials suitable for lighting reflector applications. More particularly, the present invention provides aluminum sheet materials with a textured surface and a process for making the sheet materials.
2. Description of the Prior Art
Aluminum sheets have been used in lighting applications, such as in reflector assemblies. For these applications, aluminum sheets must have various photometric properties to meet requirements of lighting fixture manufacturers. Among the most highly sought photometric properties are high total reflection (the amount of light reflected by the sheet, as a percentage of incident light), and diffuse or non-directional reflection of light. These properties have been achieved in the prior art by a combination of careful alloy selection for the sheet stock material and one of two cold rolling practices. In one practice, the sheet stock is cold rolled to a standard mill finish, followed by either chemical/electrochemical finishing or organic coating. In the other practice, at least one surface of aluminum alloy sheet is cold rolled with a specular or bright cold rolled surface using a work roll that has been ground to a smooth finish. This resultant sheet has high specular or mirror like reflectance and high total reflectance. Once bright rolled, diffuse and nonspecular light reflectance is imparted to the sheet through chemical and electrochemical (anodizing) surface treatments.
Available methods for producing a desired diffuse, non-directional reflecting surface with sufficient total reflectance have a number of drawbacks. Firstly, the mill finish cold rolled practice produces a final product finish that has substantial directional light reflecting properties due to an array of closely spaced microridges and valleys imprinted into the sheet surface in the direction of rolling. Secondly, bright cold rolled specular surfaces are costly to produce and are easily rendered unfit for use through superficial marking of the specular surface during rolling, handling, and/or final finishing. Thirdly, the generation of a diffuse, non-directional photometric surface through chemical/electrochemical finishing of the bright surface is not easily reproducible due to microstructural features in the sheet that can adversely affect consistency of the finished product. For instance, photometric properties including specular reflectance, total reflectance, DI (image clarity), color and directionality, of the finished sheet are affected by impurities in the selected aluminum alloy, the as-rolled finish, thermomechanical processing, and chemical/electrochemical finishing operations. Clear organic coatings suffer from lack of durability, as the organic coating is subject to yellowing and cracking as it ages.
Though heretofore not used in lighting applications, textured sheet stock has been produced for use in exterior auto body parts. Textured sheets are produced by imparting to one or both flat opposed surfaces of sheet stock (or strip stock) a pattern of surface features. This has been practiced to reduce and control frictional forces and/or to provide adequate lubrication between the sheet surface and tooling used during forming operations such as stamping. Textured sheet stock has also been used to prepare a surface for painting. Patterns of surface features have been imparted in sheet stock by creating a selected textured roll surface on one or more work rolls of a cold rolling mill employed to reduce the sheet stock to final thickness. Various techniques including shot blast, energy beam (such as laser or electron beam), or electro-discharge (EDT) have been used to create the selected textured roll surface.
U.S. Pat. Nos. 4,111,032 and 5,025,547; Japanese patent publications nos. 64-5628, dated Jan. 10, 1989 and 2-107751, dated Apr. 19, 1990; and publications "Electron Beam Texturing of Rolls", "Update of Developments in Lasertex Technology", "Development of High Image Clarity Steel Sheet by Laser Texturing", "Application of Laser-Texturing Steel Sheets for Auto Body Panels", and "Advance in Electro-Discharge Texturing (EDT) for Cold Mill Work Rolls" appearing in "Iron and Steel Engineer", August 1991, pages 33 through 59 inclusive, describe texturing of steel sheet stock. Further, U.S. Pat. Nos. 4,111,032 and 5,025,547 describe texturing in which the resultant textured sheet has raised portions or beads.
None of the above inventions and patents, taken either singly or in combination, is seen to describe the instant invention as claimed.